1. Technical Field
Embodiments described herein relate to a multichannel photometric measurement apparatus, for example, a detecting portion of a measurement sensor for measuring the moisture percentage or thickness of a sheet (paper) using near infrared rays.
2. Related Art
In a process for manufacturing paper, it is important to control the moisture percentage of paper. This process thus requires a moisture measurement sensor which monitors the moisture content of paper being manufactured.
Examples of the method for measuring the moisture content include a method for calculating the moisture percentage by multivariable analysis of measurement results of transmission attenuation ratios obtained using near infrared rays.
Specifically, the moisture percentage is calculated from (1) the transmission attenuation ratio of 1.94-μm wavelength light, which is significantly absorbed into water, (2) the transmission attenuation ratio of 2.1-μm wavelength light, which is significantly absorbed into cellulose constituting 80% of paper, and (3) the transmission attenuation ratio of 1.7-μm wavelength light, which is absorbed into neither water nor cellulose.
A halogen lamp is used as a light source in many cases. Light emitted from the light source passes through paper. The transmitted light is detected by an InGaAs photodiode or a PbS cell sensitive to a wavelength band of the transmitted light.
FIG. 6A illustrates an example of a photometric measurement apparatus in the related art. In this apparatus, an upper sensor head 1 and a lower sensor head 2 are reciprocally driven in the direction of arrow X in synchronization with each other while being engaged with an O-shaped frame 3. A target to be measured (paper sheet) 4 is conveyed between the upper sensor head 1 and the lower sensor head 2 by a conveying unit (not illustrated), for example, in a direction coming out of the plane of the paper of the drawing. As a result, the upper sensor head 1 and the lower sensor head 2 scan the paper sheet in a zigzag manner. Note that the paper sheet 4 being conveyed is not in contact with the upper sensor head 1 or the lower sensor head 2.
FIG. 6B illustrates the upper sensor head 1 and the lower sensor head 2 in detail. In the example illustrated in FIG. 6B, a halogen lamp 5 and a filter wheel 6 are arranged in the upper sensor head 1. A pair of reflection mirrors 8 is arranged with the paper sheet 4 sandwiched therebetween. Three kinds of light having wavelengths of 1.94 μm, 2.1 μm, and 1.7 μm, respectively, are filtered through the filter wheel 6 and then becomes incident on the reflection mirrors 8.
After that, the three kinds of light are scattered by the pair of reflection mirrors, becomes incident on a light receiving element 10 and then converted into detected signals (electric signals). These detected signals are amplified by an amplifier 12 and A/D converted. After that, a CPU 11 calculates the moisture percentage of the paper sheet 4 based on the detected signals.
Recently, a method using a semiconductor light emitting element such as an LD or LED as a light source has been used in part of the industry (see FIG. 3 of JP-T-2008-539422 “SENSOR AND METHOD FOR MEASURING SELECTED COMPONENTS IN MOVING SHEET PRODUCTS”). In comparison with a halogen lamp, the LD and LED have a longer life and a higher luminous efficiency with respect to power consumption, and can be electrically modulated using a lock-in amplifier having a high noise canceling performance.
The semiconductor light emitting element can be electrically modulated unlike the halogen lamp light source that requires a mechanical scheme for modulation. The mechanical modulation may lead to, for example, developing a failure due to wear of a portion that moves mechanically and may involve an increase in cost incurred for a mechanism used to measure a modulation frequency. Furthermore, mechanical modulation is difficult at high frequencies. Electrical modulation, on the other hand, does not develop a failure due to wear. Electrical modulation also makes it possible to find out the frequency directly based on a drive frequency. Accordingly, a mechanism to measure the frequency may be spared.
FIG. 7 illustrates another example of the related art described in JP-T-2008-539422. FIG. 7 is a schematic diagram illustrating a moisture sensor system which measures moisture content in a moving paper sheet. This system includes a measurement wavelength light source controller 42 and a reference wavelength light source controller 40. The controller 42 modulates, and controls the temperature of, a measurement light source 16. The controller 40 modulates, and controls the temperature of, a reference light source 14.
A power source 41 is connected to the controllers 40 and 42. The light sources 14 and 16 are coupled to first ends of optical fibers 23 and 24, respectively. Second ends of the optical fibers 23 and 24 are connected to an optical head 28.
A sheet (paper) 30 is arranged adjacent to the optical head 28, so that light 31 can be directed from the optical head 28 to the sheet 30.
Part of reflected light 33 is condensed by the optical head 28. The optical head 28 delivers the condensed light to a detector 34 through an optical fiber 32. In this manner, the optical fibers 23 and 24 carry the beams from the light sources 14 and 16 to the optical head 28, respectively. The optical fiber 32, on the other hand, carries the beam from the detector.
This system includes an amplifier 36, a reference wavelength lock-in amplifier 20, a measurement wavelength lock-in amplifier 18, and a computer 19 for analyzing data signals. The amplifier 36 converts photoinduced current from the detector 34 into a voltage signal and inputs the voltage signal to the measurement wavelength lock-in amplifier 18 and the reference wavelength lock-in amplifier 20. The lock-in amplifiers 18 and 20 amplify the modulated signal while converting the signal into a DC-level signal. The lock-in amplifiers 18 and 20 further pass the converted signal through a low-pass filter (not illustrated), whereby unmodulated background noise is suppressed. As a result, a low-level modulated signal is eliminated from the background.
A cutoff frequency of the low-pass filter is ten times lower than the modulation frequency. The larger the difference between the cutoff frequency and the modulation frequency, the better the noise canceling performance of the lock-in detection. Waveforms output from internal oscillators of the lock-in amplifiers 18 and 20 are used as reference waveforms for the light source controllers 40 and 42 to modulate light output from the light sources 14 and 16.
The light from the reference light source 14 and the measurement light source 16 is transmitted through the common optical fibers 23 and 24 by frequency-division multiplexing (FDM). Consequently, a multiplexer and a demultiplexer can be configured. To perform FDM, the measurement light source 14 and the reference light source 16 are modulated at different frequencies by the controllers 40 and 42, respectively.
Multiplexing allows each light source to be modulated at a different frequency. A single set of the detector 34 and the preamplifier 36, therefore, has only to be provided to detect the wavelength of light emitted from each light source.
The sensor system illustrated in FIG. 7 detects light beams reflected from the sheet (paper) 30 in a reflective mode. In addition, a detector 29 may be arranged on the opposite side of the sheet 30 to detect light beams having passed through the sheet 30. This enables the system to detect the light beams also in a transmissive mode, in which case optical elements of the detector 29 are connected to the optical fiber 32.
With this method, light output from a plurality of semiconductor light emitting elements can be measured with one detector. This is because it is possible to modulate each light emitting element at a different frequency and to separate a detected signal from each light emitting element by discriminating the frequency of a light receiving signal. When using a light source such as a halogen lamp which outputs white light, on the other hand, it is necessary to resolve the wavelength of detected light using, for example, an optical filter. Therefore, one detector is needed for each filter, leading to an increase in cost and a complicated configuration of the optical system.